FIG. 1 is a cross-sectional view of a wind turbine rotor blade 10. The blade 10 has an outer shell 12, which is fabricated from two half shells: a leeward shell 14 and a windward shell 16. The shells 14, 16 are moulded from glass-fibre reinforced plastic (GRP). Parts of the outer shell 12 are of sandwich panel construction and comprise a core 18 of lightweight foam (e.g. polyurethane), which is sandwiched between inner and outer GRP layers 20, 22 or ‘skins’.
The blade 10 comprises first and second pairs of spar caps 24, 26, 28, 30 arranged between sandwich panel regions of the outer shell 12. One spar cap of each pair 24, 28 is integrated with the windward shell 16 and the other spar cap of each pair 26, 30 is integrated with the leeward shell 14. The spar caps 24, 26, 28, 30 of the respective pairs are mutually opposed and extend longitudinally along the length of the blade 10. A first longitudinally-extending shear web 32 bridges the first pair of spar caps 24, 26 and a second longitudinally-extending shear web 34 bridges the second pair of spar caps 28, 30. The shear webs 32, 34 in combination with the spar caps 24, 26, 28, 30 form a pair of I-beam structures, which transfer loads effectively from the rotating blade 10 to the hub of the wind turbine (not shown). The spar caps 24, 26, 28, 30 in particular transfer tensile and compressive bending loads, whilst the shear webs 32, 34 transfer shear stresses in the blade 10.
Each spar cap 24, 26, 28, 30 has a substantially rectangular cross section and is made up of a stack of pre-fabricated reinforcing strips 36. The strips 36 are pultruded strips of carbon-fibre reinforced plastic (CFRP), and are substantially flat and of rectangular cross section. The number of strips 36 in the stack depends upon the thickness of the strips 36 and the required thickness of the shell 12, but typically there may be between four and twelve strips 36 in the stack. The strips 36 have a high tensile strength, and hence have a high load bearing capacity.
The strips 36 are formed by pultrusion, a continuous process similar to extrusion, in which fibres are pulled through a supply of liquid resin and through dies that shape the strip 36. The resin is then cured, for example by heating in an open chamber, or by employing heated dies that cure the resin as the strip 36 is pultruded.
The so-called ‘structural shell design’ shown in FIG. 1 in which the spar caps 24, 26, 28, 30 are integrated within the structure of the outer shell 12 avoids the need for a separate spar cap such as a reinforcing beam, which is typically bonded to an inner surface of the shell in many conventional wind turbine blades. Other examples of rotor blades having a structural shell design are described in EP 1 520 983, WO 2006/082479 and UK patent application number 1121649.6.
The wind turbine blade shown in FIG. 1 is made using a resin-infusion (RI) process, whereby the various laminate layers of the shell 12 are laid up in a mould cavity, and a vacuum is applied to the cavity. Resin is then introduced to the mould, and the vacuum pressure causes the resin to flow over and around the laminate layers and to infuse into the interstitial spaces between the layers. To complete the process, the resin-infused layup is cured to harden the resin and bond the various laminate layers together to form the blade.
The pultruded reinforcing strips described above tend to have a relatively smooth and flat outer surface, which is a feature of the pultrusion process. As a result, when the strips are stacked one on top of the other in the mould, there is very little space at the interfaces between the strips. This lack of space makes it difficult for resin to infuse between the strips, and can result in a poor bond forming between the strips. If the strips are not properly bonded together, then there is a risk of delamination occurring in the blade structure, which may lead to failure of the blade in use. This problem is not limited to pultruded strips, but may also exist when other types of reinforcing strips having a smooth outer surface are stacked.
One known method for obtaining a surface that is more suitable for bonding is to provide a ‘peel ply’ 38 on the pultruded reinforcing strip as illustrated in FIG. 2a, which can be removed to form a roughened surface 40 as shown in FIG. 2b. Such peel plies 38 are typically made of a woven fabric such as polyamide. During the pultrusion process, the peel ply 38 is drawn through a die together with the fibres and the resin. The peel ply 38 is cured onto the surface of the reinforcing strip 36 as the resin is cured. When the peel ply 38 is removed, it removes a layer of cured resin from the surface of the strip 36, thereby providing a roughened surface 40 that is free from contamination. The roughened surface 40 provides space at the interface between the stacked strips 36, allowing resin to infiltrate between the strips 36, for example by capillary action.
However, in practice, peel ply 38 cannot be applied to the entire surface of a strip 36 during the pultrusion process. In particular, peel ply 38 cannot extend to the outermost edges of the surface, since the peel ply 38 would become caught in the machinery used in the pultrusion process. A peripheral region 42 of the strip 36 must therefore be left uncovered by the peel ply 38, as is shown in FIG. 2a. This peripheral region 42 lies flush with the peel ply surface 46, so that when the peel ply 38 is removed the peripheral region 42 lies above the roughened surface 40, as shown in FIG. 2b. 
When the strips 36 are stacked, the peripheral regions 42 of neighbouring strips 36 contact one another. The smooth surfaces of the peripheral regions 42 mean that little space is left between the peripheral regions 42 of the strips 36, and resin cannot infiltrate between the surfaces at this area of contact. Thus, the area of contact between the peripheral regions 42 acts as a barrier that prevents resin infiltrating between the opposed roughened surfaces 40 of adjacent strips 36 in the stack. An insufficient quantity of resin is therefore dispersed between the strips 36, which reduces the strength of the interfacial bonds between strips 36, and can lead to delamination.
It is an object of the invention to mitigate or overcome this problem.